Add global statistics screen

[franciscogfsm]

This pull request introduces a new global statistics feature to SuperTux, allowing players to view aggregated statistics across all worlds and levels. The implementation includes new classes for statistics management, updates to the main menu, and localization support for new UI elements. It also refactors how level completion statistics are tracked and saved.

Global Statistics Feature:

• Added a new GlobalStatsManager class to aggregate and provide summary statistics (coins, secrets, dolls, levels, time, etc.) across all worlds and levels. (src/supertux/global_stats_manager.cpp, src/supertux/global_stats_manager.hpp) [1] [2]
• Introduced a new GlobalStatsMenu and GlobalStatsScreen, accessible from the main menu, to display these aggregated statistics to the player. (src/supertux/menu/global_stats_menu.cpp, src/supertux/menu/global_stats_menu.hpp) [1] [2]
• Updated the main menu to include a "Statistics" submenu linking to the new global statistics screen. (src/supertux/menu/main_menu.cpp, src/supertux/menu/main_menu.hpp, src/supertux/menu/menu_storage.cpp) [1] [2] [3]

Level Statistics Tracking Improvements:

• Refactored LevelsetScreen to track and update best statistics (coins, tuxdolls, secrets, time, perfect completion) for each level, and save these in the player's savegame. (src/supertux/levelset_screen.cpp, src/supertux/levelset_screen.hpp, src/supertux/game_session.cpp) [1] [2] [3] [4] [5] [6] [7] [8] [9]

Localization:

• Added new strings for the statistics feature to the message template and provided translations for Portuguese and Brazilian Portuguese. (data/locale/messages.pot, data/locale/pt.po, data/locale/pt_BR.po) [1] [2] [3]

Build System:

• Updated the CMake configuration to include new source files for the statistics screen and menu. (CMakeLists.txt)